Interferometers have been known and used for a long time. They are used for many purposes, including measuring characteristics of gases, liquids, and materials, through the use of transmitted or reflected light. There exist many types of interferometers that are classified by their optical design. A few of the most widely used interferometer types include Fizeau, Twyman-Green, Michaelson, and Mach-Zender. Each of these optical designs produces interference patterns called interferograms which are generated by the optical interference of test and reference wavefronts. In a typical interferometer, test and reference beams are obtained by appropriately splitting an incoming source beam (“beams” and “wavefronts” used interchangeably herein, with a “wavefront” being understood by one of ordinary skill in the art as propagating along the optical axis and sweeping out a volume that defines the light beam). One of the beams interacts with an object under test (hence commonly referred to as the “test beam”) thus carrying information about the test object being measured, while the other interacts with a known reference object (hence, commonly referred to as the “reference beam”). Interfering or otherwise coherently superimposing these two wavefronts produces an interferogram.
Information about a measured object can be extracted from a single interferogram. This technique allows for fast data acquisition, however, it typically suffers from poor spatial resolution, time consuming and complex data processing and/or non-uniform data sampling. Thus, it is often desirable to use other techniques instead. The most common techniques use three or more phase-shifted interferograms (typically three to twelve). Using multiple phase-shifted interferograms provides additional information that can be used to greatly increase the accuracy of the analysis.
Phase-shifting is a method used to change the phase between the test and reference wavefronts in a controllable way. During the last 20 years, various methods have been used to practically implement phase shifting techniques, including mechanically moving the reference object small distances comparable to the wavelength of light, or placing photo-elastic modulators and crystal retarders in the beam path. Almost all of these methods use a sequential approach (serial in time) to generate phase-shifted interferograms, which is accomplished by introducing prescribed changes to the wavefront phase while a detector acquires a series of data images. For example, the sequence of acquiring temporal phase-shifted interferograms occurs as follows: acquire interferogram, then shift the phase, acquire interferogram, then shift the phase, and so on. However, these known time-dependent methods are sensitive to environmental conditions during the span of time in which series of interferograms are acquired. Environmental conditions that can introduce errors include vibration, airflow, temperature changes, object movements, etc. Vibration is usually the major cause of error. Elaborate mounts or expensive vibration isolation tables are commonly used to isolate temporal phase-shifted interferometers from the physical environment.
To enable interferometric measurements under normal environmental conditions, without special isolation equipment, instruments have been developed to acquire multiple phase-shifted interferograms simultaneously. This eliminates or greatly reduces the effect of these errors on measurements. However, such simultaneous phase shifting methods have to date been limited to particular types of interferometers, such as the Twyman-Green or Mach-Zender types discussed below.
U.S. Pat. No. 4,583,855 (issued to Barekat) entitled “Optical Phase Measuring Apparatus” relates to use of a polarization type Twyman-Green interferometer with quarter-waveplates and polarizers. (“Quarter-waveplates” and “half-waveplates” used herein are understood by one of ordinary skill in the art as equivalent to quarter-wave retarders and half-wave retarders, respectively). Koliopulos in a paper entitled “Simultaneous Phase Shift Interferometer”, Proc. SPIE Vol. 1531, p. 119 (1992), described the use of a polarization type Twyman-Green interferometer. A. Hettwer, J. Krantz and J. Schwider in a paper titled “Three Channel Phase-Shifting Interferometer Using Polarization Optics and A Diffraction Grating” Opt. Eng., 39(4) (April 2000) described a Twyman-Green interferometer. German Patent DE 196,52,113, A1 awarded to J. Schwider discloses the invention that is described in his above-cited paper, based on a Twyman-Green interferometer. U.S. Pat. No. 6,304,330 entitled “Method and Apparatus for Splitting, Imaging and Measuring Wavefronts in Interferometry” and U.S. Pat. No. 6,552,808 are directed to a modified polarization type Mach-Zender and Twyman-Green interferometers.
As intimated above, optical interferometers are typically constructed of optical components such as lenses, mirrors, beamsplitters, and waveplates. These components usually have slight imperfections or deviations from an ideal perfect component. From a practical standpoint, Twyman-Green type interferometers can suffer from a configuration having a reference arm and a test arm that are of separate paths. Because the interferogram generated by the interferometer is an image or pattern that registers differences between the test and reference wavefront, a separation of the test and reference path such as in a Twyman-Green type interferometer, can cause imperfections and aberrations in the optical components encountered in one path, but not in the other path, to register as measurement errors. That is, where the beam paths are separate, an error in one path not present in the other path can register in the final comparison result (the interferogram). Because the aforementioned interferometers have Twyman Green type configurations, they are susceptible to the disadvantages of separate paths between the test and reference beams.
A well recognized advantage of a Fizeau interferometer is the feature of a common path shared by the test and reference wavefronts throughout most of the interferometer. Where the test and reference wavefronts both travel through the same optical components, imperfections and aberrations in components are common to both wavefronts, and do not register as measurement errors in the interferogram. Thus imperfect components do not impart “difference errors” in the final comparison of the test object to the reference object. As such, the Fizeau configuration is significantly more tolerant and robust compared to other interferometry systems. Imperfect components in its construction have little or no effect on the accuracy and precision of the final measurement results. This and other typical features of the Fizeau, including an alignment mode, ability to measure large flat optics, zoom capabilities, and ease of use with corrective null optics, have made the Fizeau a very popular, if not the most popular, interferometer configuration for practical applications.
However, despite such advantages of the Fizeau-type interferometers, there has been little, if any, ability or method known to construct or use a Fizeau interferometer that is capable of simultaneous phase-shifting.
Accordingly, there is a desire for a Fizeau-type interferometer capable of simultaneous phase shifting, and, further, for a simultaneous phase shifting Fizeau-type interferometer that uses orthogonally polarized beams.